1. Field of the Invention
The present invention relates to a headbox tube bank apparatus and method for improving the flow therethrough of stock in a papermaking machine. More particularly, the present invention relates to a headbox tube bank apparatus for improving the uniformity, stability, cleanliness, and lowering the turbulence of the stock during flow thereof through each tube and nozzle thereof.
2. Information Disclosure Statement
In the papermaking art, paper pulp, otherwise known as stock or furnish, is ejected under high pressure from a headbox onto a moving screen. Water drains from the stock through the screen such that a web of paper is formed on the upper surface of the screen.
More particularly, the headbox includes a slice chamber having an upstream and a downstream end. The downstream end of the slice chamber defines a slice lip which is adjustable such that the curtain of stock ejected from the headbox may be adjusted so that the stock contacts the screen at the optimum angle and with a relatively uniform thickness in a cross-machine direction.
The stock flows into the slice chamber through the upstream end of the slice chamber. Such upstream end is connected to a source of high pressure stock by means of a plurality of tubes arranged such that the pressurized stock flows through the tubes and is evenly distributed into the slice chamber through the upstream end thereof.
Many headboxes have a width in the region of 30 or more feet, and the problem of evenly distributing the pressurized stock through the plurality of tubes or tube bank is considerable.
Several proposals have been made in an attempt to improve the flow stability of the stock flowing through a tube bank. Additionally, attempts have been made in an effort to lower the turbulence of the stock in high speed applications.
Also, in high speed applications, ways have been sought in an attempt to improve the dispersion of fibers within the stock or to evenly distribute the fibers flowing through the tube bank.
Another important feature that is required in a headbox is cleanliness of the headbox such that flow velocities are adequate enough to avoid the collection therein of fibers and contaminants.
Accordingly, the present invention provides a headbox tube bank in which a downstream end thereof has a maximum open area where the stock flows from the tube bank into the slice chamber. Such high open area outlet improves the flow stability of the stock and inhibits the development of undesirable secondary motions or eddies that could otherwise develop in the wake of the main flow.
However, in order to maintain the aforementioned cleanliness within a headbox, it is essential that the stock flow at an extremely high velocity through the individual tubes or nozzles of the tube bank.
Accordingly, it was determined that the tubes should have a relatively small height at the downstream end thereof.
Furthermore, according to the present invention, the tube bank includes a tube design where the flow would change from a circular cross-section at the upstream end of the tube to one of a rectangular cross-section at the downstream end of the tube. Also, the height of the rectangular portion is small compared to the cross-machine directional width of the downstream end or outlet.
However, under such conditions, experimentation indicated that the flow of stock would not normally expand uniformly when undergoing a transition from a circular to a rectangular cross-section. Typically, such flow tends to attach to one or other of the side walls of the rectangular portion, thereby creating a high velocity jet down one side of the tube.
Additionally, another problem that was difficult to resolve was the design of a tube of the aforementioned type that would be relatively easy to manufacture and sufficiently rugged to withstand the relatively hostile environment existing within a headbox of a papermaking machine.
In order to overcome the aforementioned problems, it was determined that in order to achieve a uniform flow in a rectangular duct, the stock flow should first expand from a circular section to a substantially square section. The stock flow would then converge to a higher aspect ratio rectangular section. The term "a high aspect ratio rectangular section" means, according to the present specification, a rectangular section which is relatively wide in a cross-machine direction and relatively low in height.
Although the aforementioned tube design was theoretically correct, such designs were found to be difficult to manufacture and lacked the necessary rigidity.
The aforementioned problem was overcome by the provision of a tube having an initial circular cross-section along the upstream end thereof, such tube having a downstream end of rectangular shape through convergence rather than expansion at the downstream end thereof.
Such convergence of the tube was found to promote a flatter velocity profile and to lower the turbulence level, characteristics that were desirable in a headbox tube bank.
The aforementioned tubes were found to be relatively easy to manufacture through hydraulically pressing a standard tube of the type used in a CONCEPT III headbox. The tube was pressed in an external dye. CONCEPT III is a common law mark of Beloit Corporation.
Moreover, the tubes were arranged in rows with each tube of a row being closely spaced relative to an adjacent tube. The rows were aligned relative to each other in the height direction and adjacent rows were arranged to define therebetween a dove-tail slot for the anchoring therein of an upstream end of a trailing element disposed within the slice chamber.
Also, such trailing elements or vanes defined an increased thickness adjacent to such dove-tail slot such that the stock flow would not slow down on entering the slice chamber.
The aforementioned geometry and trailing element design was found to maintain a high tube and nozzle flow velocity for maintaining cleanliness while promoting a more stable flow therethrough by not allowing significant secondary motion or flow within the main stock flow to occur.
Therefore, it is a primary objective of the present invention to provide a headbox tube bank apparatus which overcomes the aforementioned inadequacies of the prior art arrangements and which makes a considerable contribution to the papermaking art.
Another object of the present invention is the provision of a headbox tube bank apparatus in which each tube defines an upstream portion having a substantially circular configuration such that although the mass flow rate remains constant throughout the tube, the velocity changes in accordance with the change in cross-sectional area along the tube. The aforementioned arrangement maximizes the velocity of the stock flowing through the tube and a downstream orifice which has a substantially rectangular configuration for progressively improving the uniformity, stability, cleanliness, and for lowering the turbulence of the stock during flow thereof through the tube and nozzle thereof.
More particularly, the inlet is circular in order to improve strength and cleanliness. Additionally, the relatively small diameter of the tube increases the velocity of the stock, thereby creating sufficient resistance for cross-machine uniformity.
Another object of the present invention is the provision of a headbox tube bank apparatus in which a frame defines a plurality of openings for supporting each tube of the tube bank so that the openings are disposed in vertically spaced rows, each adjacent row defining therebetween a dove-tail shaped slot for the reception therein of a trailing element.
Another object of the present invention is the provision of a headbox tube bank apparatus in which each trailing element has an increase in thickness immediately downstream relative to an anchoring portion disposed within a dove-tail shaped slot for optimizing the stability, dispersion, and cleanliness of stock flow immediately downstream relative to the downstream orifice of each tube.
Another object of the present invention is the provision of a headbox tube bank apparatus in which each tube includes an upstream portion having an upstream and a downstream end, the upstream portion having a circular configuration from the upstream to the downstream end.
Another object of the present invention is the provision of a headbox tube bank apparatus in which each tube includes a downstream portion having an upstream length of substantially uniform flow area in a cross-machine direction and a downstream length having a substantially rectangular flow area which decreases along the length thereof in a flow direction.
Other objects and advantages of the present invention will be apparent to those skilled in the art by a consideration of the detailed description contained hereinafter taken in conjunction with the annexed drawings.